A stage micrometer is the term typically referring to a slide (1" x 3" microscope) that comes with a scale on its
surface. The sides are mounted with a reticle scale that is used for calibrating the reticles of the eyepiece as well as the
objective powers. Although most stage micrometers are made of glass, they are
also composed of metal especially for dissecting microscopes.
While microscopes serve the primary function of
enlarging objects being viewed, they are also used to make measurements of the
objects/specimen. Given that this is not possible with the ordinary ruler, the
eyepiece reticle (eyepiece scale) is used to make such measurements.
micrometers are particularly useful given that the objectives and eyepiece reticles
of a microscope are often interchanged. For this reason, there is a need to
carry out a routine calibration to ensure accuracy when measuring
*A micrometer may be used by being directly
mounted on the object being viewed. However, this is expensive and impractical
and thus not common.
Eyepiece Graticule/Reticule Calibration
The eyepiece graticule is an important part of the
eyepiece that allows for measurements. It's a disc made of glass marked with a
scale from 0 to 100 (um). Depending on the type of eyepiece, the eyepiece
graticule may be fitted onto the eyepiece or be purchased separately and
attached onto the eyepiece for measurements.
During calibration, it's
important that each individual lens be calibrated separately starting with the
lowest power objective. This makes it possible to have distinct calibration
power for each.
During the calibration process, the stage
micrometer has to be aligned with the eyepiece graticule scale followed
by taking the readings. Readings obtained from the scales are then used for the
purposes of calculating the calibration factor. The process involves aligning
the zero point/lines on both scales first.
Once the two points have been aligned
properly, then the one calibrating has to scan through the scale lines to find
a point where the lines of both scales align. It's important to ensure that
the starting point (zero point/line) of the two scales align properly before
scanning to find the next lines. This ensures that the right
readings are obtained and recorded for calculating the calibration factor.
calculate the relationship between the two points that have aligned, the following formula is used:
Number of units = number of divisions on stage
micrometer divided by the number of divisions on the eyepiece.
Assuming that in a given alignment there are 30
divisions on the stage micrometer that has aligned with 10 on the eyepiece
scale, calculating this would give us 3 units. In the event that the calculation gives a number with
decimals (e.g. 3.6363) then the number may be rounded off. For instance, 3.6363
can be rounded off to 4 or 3.6. Here, the number obtained from the calculation
is the calibration factor and gives the number of units in each division of the
Here, it's worth noting that the conversion
factor of each objective is different. For this reason, it's important to
calibrate each objective lens individually so as to obtain their respective conversion
factor. This becomes important particularly when making measurements when viewing
the specimen using different magnifications.
When making measurements, it's particularly
important that the right units of measurements are used. Here, 1 mm is equal to
1000 micrometers (um). The figures obtained from the calculations should be
converted to micrometers during measurements.
When measuring the size of the object/specimen,
it's always important to measure the diameter. Here, the student may measure
the longest and shortest diameter of the specimen in the field of view. When in
the field of view, the student should calculate the eyepiece divisions, which
represents the diameter of the specimen.
For instance, the diameter of the
specimen here may be 12 divisions. This number does not really represent any
In order to determine the length of the specimen, these units
should be multiplied to the conversion factor in order to get the measurements
in micrometers. This makes it possible to tell the actually length/width of the
specimen/object being observed.
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